The present invention is directed to a device which enables correcting a lateral position of a sheet of material, such as a paper sheet or a box board sheet, as it is taken from a pile and before it is fit into a machine for further processing.
Certain machines, such as, for instance, printing or blank cutting machines, require a high position accuracy of a sheet being fed thereto, although the sheets are supplied in piles not always strictly vertical or else the pile is not always moved up to an infeed station exactly along a machine axis. Such machines usually include two stations upstream with regard to the grippers which seize the sheet one-by-one. These two stations include a feeder table which is preceded by a feeding or infeed station.
On the feeder table, a single sheet is simultaneously aligned on its front and lateral edges. The lateral alignment is achieved by means of two rollers nipping the edge and pulling the sheet sideways through a maximum distance of 8 mm. This maximum distance of 8 mm, thus, represents the maximum misalignment which is allowed for the sheet infeed on the feeder table through the infeed station.
The infeed station includes a pallet on which the sheets are piled up or on which are loaded with a pile previously built up owing to an intermediary, movable pallet. The four corners of the pallet are suspended on chains to enable the pallet to be hoisted during the departure of the sheets into the machine so that the top sheet on the pile remains at a constant level, as monitored by a photo-electric cell which is arranged adjacent the upper plane which is desired. This infeed station, moreover, includes a pickup unit which is arranged immediately above the plane of the top sheet. The purpose of this unit is to initially assure the lifting of the uppermost sheet solely and then its infeed into the feeder table.
To achieve these operations, the pick-up unit must, first, have lifting appliances arranged along a bar parallel to the upper sheet's front edge. In addition, the pick-up unit includes two more lifting suction cups designed for descending and being pressed onto the sheet, for seizing the sheet by means of an air vacuum built up within the cups, and for raising the sheet. For easy lifting of the sheet's front edge, two nozzles blow air against the sheet's rear edge. As soon as the sheet begins to lift off, a cam-actuated blower foot is applied on the rear edge of the sheet beneath the sheet being lifted.
The pick-up unit, secondly, is implemented with a carrier suction cup arranged on a crosswise bar perpendicular to the sheet travel direction and situated approximately in the sheet center area. This bar is held on the sheet of the pick-up unit by jointed or articulated arms. These arms are driven by the main machine's motor through a mechanical transmission assembly, which includes chains, cardan joints and cams, which are constructed and engineered in such a way that they can move the arms forward in order to push the sheet onto the feeder table, as required by the machine's operating cycle. Prior to this movement, the lifting suction cup will have released the sheet.
The frame of the pick-up unit is ordinarily mounted on the infeed station in such a way that its position, both vertically and in the sheet traveling direction, can be specifically adjusted to a pile of board or paper sheets of a given dimension.
As may be gathered from this arrangement, the precision of the sheet infeed position depends, to a large extent, on the initial pile position at this infeed station.
To this aim, various devices for pile position correction have, up to now, been developed. For instance, for this purpose, devices now being fitted by the assignee of the applicant on his machines, which device includes, on the one hand, a lever which, on one end, has a roller that follows successive positions of the upper sheets of the pile, whereas the other end actuates, when necessary, one of two switches corresponding to the pile shift to the right or left of the desired position. On the other hand, this device comprises an electric motor movable along a vertical column, which extends close to the lateral pallet edge and the threaded outlet axle of the motor is engaged in an aperture of the pallet. In this way, the motor moves along the column following the pallet and, depending on whether one or the other of the mentioned switches is switched on, turns the threaded axle in either a clockwise or counterclockwise direction, which will shift the pallet sideways in the desired direction to move it towards the desired orientation with respect to the infeed station.
According to a second device, which is described in U.S. Pat. No. 4,245,830, whose disclosure is incorporated by reference thereto, a mechanism for checking the registry of the upper lateral edge of the pile consists essentially of a sliding shoe provided with a metal flag activating two proximity detectors which are arranged horizontally, side-by-side. In this way, if the upper edge of the pile is in the desired position, the flag will have no influence on either of the detectors. However, if the pile has been shifted from the desired position, the flag will influence one of the detectors, which will simultaneously cause an electric motor to be actuated to adjust the lateral position of the pile to correct the detected misalignment.
These devices, which are designed for correcting the lateral pile position, have several drawbacks. In fact, the control of the upper pile edge subjected to a given measurement depends essentially on the repeatability of the detector switchoff, which might be jeopardized, by mechanical switches being either rubbed or being squeezed during the sensor movement. If, on the other hand, production speed is expected to be increased, beginning with a voluminous and large pile which has excessively been displaced, the motor is likely to have inadequate power to overcome the pile's mass inertia, which inertia will prevent the motor from correcting the position sufficiently quickly. Finally and especially, the sensors measure, generally, the medium position of the five or ten uppermost sheets of a pile, but do not detect a large, though exceptional, displacement of one of them causing substantially a full standstill of a single sheet of the group and this single misaligned sheet can cause a faulty alignment.